1. Field of the Invention
This invention relates generally to bearing systems which isolate structures from motions produced by dynamic loads and, more particularly, it relates to a rocking hinge bearing system which inhibits instability of the structure when subjected to dynamic loads including seismic, wind, vehicle impact, or all other transient loads.
2. Description of the Prior Art
Bents for structural frames usually consist of a cap beam supported by multiple columns or a single pier wall to resist lateral loads transversely (in the plane of the bent) and longitudinally (perpendicular to the plane of the bent). Pier walls are stiffer in the transverse direction because they act as shear walls in that direction. Likewise, multiple column bents are generally stiffer in the transverse direction because the frame action in the plane of the bent is usually more rigid than the longitudinal frame action.
In the past, many devices have been created to soften dynamic excitation by either isolating the structure from the force of dynamic excitation or dissipating and absorbing energy. Included among these devices are laminated elastomeric bearings which typically consist of rubber or resilient pads laminated between steel shims. While the laminated elastomeric bearings support the axial load of the structure and will, to some extent, attenuate the motion of the structure, these bearings are too flexible and have a low shearing resistance which limits their ability to take large lateral loads without displaying excessive horizontal deflections or failing in shear.
Another type of device to soften dynamic excitation is laminated lead-rubber bearings. The laminated lead-rubber bearings are similar to the laminated elastomeric bearings except that the laminated lead-rubber bearing has a lead core to stiffen the horizontal movement of the bearing and to better maintain the integrity of the resilient pads. However, the lead core reduces the opportunity for the bearing to recenter itself after being subjected to a horizontal load because the remaining inertial or static forces within the structural system may not be large enough to deform the lead core back to its original configuration.
Other types of bearings include friction pendulum bearings, steel hysteretic dampers, hydraulic dampers, and lead or rubber extrusion dampers. From an economic viewpoint, it is desirable to avoid the use of bearings by using monolithic construction that incorporates columns that are capable of developing plastic hinges where they connect to the rest of the structure. Unfortunately, conventional plastic hinge columns are only average performers in both hard soils/rocks and soft soils whereas some of the seismic isolation bearings have a clear advantage in either hard soils/rocks or soft soils. In particular, lead rubber bearings seem to have a clear advantage in soft soils but under perform in hard soils/rocks. Hence, the indiscriminate use of seismic isolation devices can lead to reduced performance.
Accordingly, there exists a need for bearing systems with natural recentering capabilities which can isolate structures from ground motions produced by dynamic loads. Additionally, a need exists for a structurally stable bearing system that can limit the moment transfer from a column to its supports in order to reduce structural damage as the structure rocks during a dynamic excitation.